Loop heat pipe

ABSTRACT

A loop heat pipe includes an evaporator to vaporize a working fluid due to heat supplied from an external heat source; a condenser to cause the vaporized working fluid to condense; and connecting lines to connect the evaporator and the condenser in a loop, wherein the evaporator includes a first space defined by a set of walls including a contact wall that comes into contact with the external heat source, a second space provided adjacent to at least one of the walls other than the contact wall, and a through-hole formed in a dividing wall separating the first space and the second space to allow the first space and the second space to communicate with each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of International Application PCT/JP2010/066329 filed on Sep. 21, 2010 designating the United States, which International application further claims the benefit of the earlier filing date of Japanese Patent Application No. 2010-075443 filed in Japan on Mar. 29, 2010, the entire contents of the International Application and the priority Japanese Application being incorporated herein by reference.

FIELD

The embodiments discussed herein relate to a loop heat pipe used to cool heat generating components such as electronic devices.

BACKGROUND

A loop heat pipe is known as a device for cooling various types of heat generating components, in which an evaporator and a condenser are connected in a loop via a vapor transport line and a liquid transport line. Liquid-phase working fluid evaporates in the evaporator due to heat supplied from an external heat source, and the vaporized working fluid is transported via the vapor transport line to the condenser, in which the vapor condenses back to a liquid by releasing heat. See, for example, Japanese Laid-open Patent Publication No. 2004-218887.

FIG. 1A through FIG. 1C illustrate a conventional evaporator 1000, where FIG. 1A is a cross-sectional view of the evaporator along the direction of flow of the working fluid, and FIG. 1B and FIG. 1C are cross-sectional views taken along the A-A′ line of FIG. 1A. A heat generating component 1010 such as an electronic device 1010 is generally shaped flat, and accordingly, the heat-receiving face 1002 of the evaporator 1000 of a loop heat pipe is made flat so as to be kept in stable contact with the heat generating component 1010. To improve the cooling ability of the loop heat pipe, it is desired to increase the internal volume of the evaporator 1000 as much as possible. On the other hand, there is another demand for shaping the evaporator to be as compact as possible. To satisfy both demands, a flat plate loop heat pipe is used.

In order to efficiently remove heat from the heat generating component 1010 during operation, it is desired to cause the working fluid 1006 supplied through the liquid transport line 1003 to the evaporator 1000 to vaporize in an efficient manner. In this regard, a wick 1007 is provided in an evaporator case 1001 so as to be in close contact with the inner wall of the evaporator case 1001. Heat is transferred promptly from the evaporator case 1001 to the wick 1007 and it allows the working fluid 1006 penetrating the wick 1007 to vaporize quickly. The vaporized working fluid is guided through grooves 1005 toward the vapor transport line 1004. However, as the heat is transferred to the evaporator 1000, the temperature of the internal working fluid rises and the adhesion between the evaporator case 1001 and the wick 1007 is degraded. This state is illustrated in FIG. 1C.

In FIG. 1C, when the saturated vapor pressure of the working fluid exceeds the atmospheric pressure at the operating temperature of the loop heat pipe, the internal surface of the evaporator case 1001 is pressed outward by the internal pressure of the working fluid. If the loop heat pipe is placed at ordinary temperature and pressure, and if the boiling point of the working fluid (such as pentane, R141B, butane, or ammonia) used in the loop heat pipe is above room temperature under standard atmospheric pressure, then the flat evaporator case 1001 deforms outward. If the evaporator has a cylindrical shape, the internal pressure is equally distributed in the circumferential direction and expansion of the evaporation case is less. In contrast, with the flat plate loop heat pipe, the internal pressure is applied toward the top face with a large area size, which causes the top wall to swell as illustrated in FIG. 1C. Especially when a flat plate evaporator is employed for the purpose of reducing the size and the weight of electric equipment, the evaporator body is made as thin as possible. In this circumstance, it is difficult to guarantee a thickness of the evaporator case 1001 enough to provide rigidity to resist the internal pressure. When the evaporator case 1001 swells due to the increasing internal pressure, adhesion between the evaporator case 1001 and the inner wick 1007 is degraded. This issue becomes more conspicuous at the top face of the evaporator case 1001 than the bottom face secured to the heat generating component 1010 (such as a CPU). Besides, a gap 1020 is produced between the evaporator case 1001 and the wick 1007 at a higher temperature. In this state, sufficient heat cannot be transferred from the evaporator case 1001 to the wick 1007, and the working fluid is prevented from vaporizing from the surface of the wick 1007. Consequently, the cooling capacity is lowered.

It is desired for the loop heat pipe to maintain thermal contact between the evaporator case 1001 and the wick 1007 during operation to ensure the thermal performance even if the temperature and the pressure of the working fluid are increasing in the evaporator.

SUMMARY

According to an aspect of the embodiments, a loop heat pipe includes:

an evaporator to vaporize a working fluid due to heat supplied from an external heat source;

a condenser to cause the vaporized working fluid to condense; and

connecting lines to connect the evaporator and the condenser in a loop,

wherein the evaporator includes

a first space having a set of walls including a contact wall that comes into contact with the external heat source,

a second space provided adjacent to at least one of the walls other than the contact wall, and

a through-hole formed in a dividing wall separating the first space and the second space to allow the first space and the second space to communicate with each other.

The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive to the invention as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a cross-sectional view of a conventional flat plate evaporator used in a loop heat pipe along a direction of flow of working fluid;

FIG. 1B is a cross-sectional view of the evaporator taken along the A-A′ line of FIG. 1A, illustrating the non-operating state;

FIG. 1C is a cross-sectional view of the evaporator taken along the A-A′ line of FIG. 1A, illustrating an issue arising in the conventional flat plate evaporator under application of heat;

FIG. 2 is a general view of a loop heat pipe to which the present invention is applied;

FIG. 3A is a cross-sectional view of an evaporator according to the first embodiment of the invention, along a direction of flow of working fluid;

FIG. 3B is a cross-sectional view of the evaporator taken along the A-A′ line of FIG. 3A;

FIG. 4 is a diagram illustrating saturated vapor pressure curves of several working fluids as a function of temperature;

FIG. 5A is a schematic cross-sectional diagram of the evaporator in the non-operating state, for explaining the advantage of the first embodiment;

FIG. 5B is a schematic cross-sectional diagram of the evaporator under application of heat, for explaining the advantage of the first embodiment;

FIG. 6A illustrates an example of mounting the evaporator according to the first embodiment;

FIG. 6B is a perspective view of the mounted evaporator of FIG. 6A;

FIG. 7 is a graph illustrating an advantage of the loop heat pipe using the evaporator according to the first embodiment;

FIG. 8A illustrates a first modification of the evaporator of the first embodiment, which evaporator is in the non-operating state;

FIG. 8B illustrates the evaporator of the first modification illustrated in FIG. 8A, which is in the operating (heat absorbing) state;

FIG. 9A illustrates a second modification of the evaporator of the first embodiment, which evaporator is in the non-operating state;

FIG. 9B illustrates the evaporator of the second modification illustrated in FIG. 9A, which is in the operating (heat absorbing state);

FIG. 10A is a cross-sectional view of an evaporator according to the second embodiment along the direction of flow of the working fluid;

FIG. 10B is a cross-sectional view taken along the A-A′ line of FIG. 10A;

FIG. 11A is a schematic diagram for explaining an advantage of the evaporator of the second embodiment, which evaporator is in the non-operating state;

FIG. 11B is a schematic diagram for explaining the advantage of the evaporator of the second embodiment, which is in the operating (heat absorbing) state;

FIG. 12A illustrates a modification of the evaporator of the second embodiment, which evaporator is in the non-operating state;

FIG. 12B illustrates the evaporator of the modification, which is in the operating (heat absorbing) state;

FIG. 13A illustrates an example of mounting the evaporator according to the second embodiment;

FIG. 13B is a perspective view of the mounted evaporator of FIG. 13A; and

FIG. 14 is a graph illustrating an advantage of the loop heat pipe using the evaporator of the second embodiment.

DESCRIPTION OF EMBODIMENTS First Embodiment

FIG. 2 illustrates an overall structure of a loop heat pipe 1 to which the present invention is applied. The loop heat pipe 1 includes an evaporator 10 which vaporizes a liquid-phase working fluid due to heat supplied from a heat generating component (e.g., an electronic component), and a condenser 11 that causes a vapor-state working fluid to condense by removing the heat. The evaporator 10 and the condenser 11 are connected in a loop by a vapor line 14 for transporting the vaporized working fluid from the evaporator 10 to the condenser 11 and a liquid line 13 for transporting the liquid-state working fluid from the condenser 11 to the evaporator 10. The liquid line 13 and the vapor line 14 form the connecting lines. In the example of FIG. 2, a blast fan 12 is provided near the condenser 11 to enhance removal of heat.

The working fluid in the vapor line 14 or the liquid line 13 is not necessarily 100% vapor or 100% liquid, and it is in a vapor phase and a liquid phase mixed with each other. During operation of the loop heat pipe 1, for the most part the working fluid inside the vapor line 14 is in the vapor phase, while for the most the working fluid inside the liquid line 13 is in the liquid phase. In this regard, the connecting lines are named the “vapor line” and the “liquid line” for the sake of convenience.

FIG. 3A and FIG. 3B illustrate an evaporator 10 according to the first embodiment of the invention, where FIG. 3A is a cross-sectional view of the evaporator 10 along a direction of flow of the working fluid and FIG. 3B is a cross-sectional view taken along the A-A′ line in FIG. 3A. In the first embodiment, the evaporator 10 has a vaporization chamber (a first space) 40A having a liquid supply path 46, and a pressure adjusting chamber (a second space) 40B for adjusting the pressure in the vaporization chamber 40A. A pressure adjusting hole 55 is formed in a dividing wall 51 that separates between the vaporization chamber 40A and the pressure adjusting chamber 40B to allow the vaporization chamber 40A and the pressure adjusting chamber 40B to communicate with each other.

In the example illustrated in FIG. 3A and FIG. 3B, the bottom face of an evaporator case 40 is a heat-receiving surface 42. The evaporator 10 is mounted on a heat generating component such that the heat receiving surface 42 comes into contact with the heat generating component such as an electronic component (see FIG. 6A) to receive heat from the electronic component. A wick (porous material) 47 is provided in the vaporization chamber 40A so as to be kept in mechanical and thermal contact with the inner wall of the vaporization chamber 40A. The (liquid-state) working fluid is supplied through the liquid line 13 into the vaporization chamber 40A and penetrates into the wick 47. The liquid absorbed in the wick 47 is heated by heat transferred from the evaporator case 40 to the wick 47. The inner space of the evaporator 10 is maintained at a saturated vapor pressure of the working fluid. When the temperature of the working fluid has reached the boiling point under the saturated vapor pressure, the working fluid is vaporized. During vaporization, the working fluid takes in latent heat. The vapor that has taken in the latent heat passes through grooves (vapor discharge grooves) 45 and flows into the vapor line 14. Simultaneously, a portion of the vapor passes through the pressure adjusting hole 55 and flows into the pressure adjusting chamber 40B. Consequently, the pressures in the vaporization chamber 40A and the pressure adjusting chamber 40B become almost the same. The saturated vapor pressure within the utilized temperature range of the working fluid 49 is at or above the atmospheric pressure in the environment in which the loop heat pipe 1 is used.

Explanation is made of the exemplified structure of the evaporator 10 illustrated in FIG. 3A and FIG. 3B. The evaporator case 40 is a flat plate case with an entire height of 18 mm, a width of 60 mm and a length of 70 mm. A double chamber structure is employed in which the pressure adjusting chamber 40B is provided on the top of the vaporization chamber 40A. The pressure adjusting chamber 40B has a space with the dimensions 66 mm length×56 mm width×1 mm height. The pressure adjusting chamber 40B and the vaporization chamber 40A are separated from each other by a dividing wall 51 with a thickness of 2 mm. A pressure adjusting hole 55 with a diameter of 1 mm is formed in the dividing wall 55 so as to allow the pressure adjusting chamber 40B to communicate with the vapor side of the vaporization chamber 40A. The inner dimensions of the vaporization chamber 40A are 66 mm length×56 mm width×11 mm height. The thickness of the walls defining the vaporization chamber 40A is 2 mm on the whole.

The material of the vaporization case 40 and the dividing wall 51 is oxygen-free copper in the first embodiment. Conventional flat evaporators are often made of a rigid material such as stainless so as to be tolerant of high internal pressure. In contrast, the evaporator of the first embodiment does not necessarily use a rigid material, as will be described below. Rather, a material with a higher thermal conductivity than stainless is used such that the temperature distribution of the evaporator case 40 becomes uniform. For example, aluminum alloy can be used for reducing weight.

The wick 47 arranged inside the vaporization chamber 40A is made of sintered nickel.

The porous diameter is about 10 μm, and the porosity is about 50%. The outer dimensions of the wick 47 are 50 mm length×56 mm width×11 mm height. Especially, the height of the wick 47 is set precisely such that the wick 47 is held in the vaporization chamber 40A in close contact with the inner wall thereof. Fifteen grooves (vapor passages) 45 with a width of 1 mm and a depth of 2 mm are formed at a pitch of 3 mm in the top face and the bottom face (which come into contact with the ceiling and the bottom of the vaporization chamber 40A, respectively). In the center of the wick 47 is formed a liquid supply path 46 with a height of 3 mm, a width of 40 mm and a length of 40 mm to take the working fluid 49 supplied from the liquid line 13 into the wick 47.

The vapor line 14 and the liquid line 13 connecting the evaporator 10 and the condenser 11 are copper pipes with an outer diameter of 6 mm, an inner diameter of 5 mm, and a length of 300 mm. The condenser 11 is also a copper pipe, like the vapor line 14 and the liquid line 13, with an outer diameter of 6 mm, an inner diameter of 5 mm and a length of 400 mm. Radiation fins are thermally connected to the circumference of the pipe, and are cooled by the blast fan 12 (see FIG. 2).

Although in the first embodiment n-pentane is used as the working fluid 49, other fluids with high saturation pressures including butane and ammonia can be used.

FIG. 4 is a graph of saturation pressure curves of various fluids. When n-pentane is used as the working fluid 49, the boiling point at atmospheric pressure is about 36° C. During operation of the loop heat pipe 1, the temperature of the working fluid 49 becomes near 50-70° C. If butane or pentane is used as the working fluid 49, the saturation pressure of the working fluid exceeds the atmospheric pressure in the temperature range of 50-70° C. With the conventional evaporator illustrated in FIG. 1A, the top wall of the case 1001 swells due to the internal pressure of the working fluid as illustrated in FIG. 10. The contact between the evaporation case 1000 and the wick 1007 is degraded and the cooling performance lowers. In contrast, with the evaporator 10 of the first embodiment with the double chamber structure, a pressure adjusting chamber 40B is provided on the top of the vaporization chamber 40A, and a pressure adjusting hole 55 is formed in the dividing wall 51 to allow the vapor coming from the surface of the wick 47 to flow into the pressure adjusting chamber 40B. The internal pressures in the vaporization chamber 40A and the pressure adjusting chamber 40B become equal.

FIG. 5A and FIG. 5B are diagrams to explain an advantage of the first embodiment. When butane is used as the working fluid 49, the vapor pressure inside the vaporization chamber 40A increases as the working fluid absorbed in the wick 47 is heated by heat transferred from the electronic component 20. Because the vaporized working fluid flows into the pressure adjusting chamber 40B through the pressure adjusting hole 55, the vapor pressure applied to the dividing wall 51 from the vaporization chamber 40A becomes equal to the vapor pressure applied to the dividing wall 51 from the pressure adjusting chamber 40B. Accordingly, the dividing wall 51 with a surface which is in contact with the wick 47 is prevented from deforming due to the internal pressure. On the other hand, the top wall 53 of the evaporator case 40 (which is also the top wall of the pressure adjusting chamber 40B in the first embodiment) expands and bends outward because the saturated vapor pressure of butane is higher than the atmospheric pressure. Even if the internal pressure in the vaporization chamber 40A becomes high due to the increasing vapor pressure of the working fluid 49, the thermal contact between the vaporization chamber 40A and the wick 47 can be maintained satisfactorily because of no deformation in the dividing wall 51.

FIG. 6A and FIG. 6B illustrate a structure in which the evaporator 10 of the first embodiment is mounted over a heat generating component. The evaporator 10 of the loop heat pipe 1 is placed, via thermal grease 21, over the electronic component 20 on a printed circuit board 30 and secured to the printed circuit board 30 using attachment screws 31.

The amount (rate) of heat absorption of the evaporator 10 is about 60 W in the first embodiment. At this time, the condenser 11 (not shown in FIG. 6A and FIG. 6B) is cooled at the room temperature (25° C.) using a blast fan 12 (90 mm diameter, 12-volt driving voltage).

FIG. 7 is a diagram illustrating the cooling ability of the loop heat pipe of the first embodiment, with a comparison example a loop heat pipe using a conventional evaporator illustrated in FIG. 1. The horizontal axis of the graph represents amount of heat generated by a heater (i.e., the electronic component), and the vertical axis represents thermal resistance [° C./W] between the evaporator 10 and the condenser 11. The thermal resistance indicates a difference between the temperature of the heat-receiving surface 42 of the evaporator 10 and the average temperature of the condense 11 per watt (divided by the quantity of heat generated by the electronic component 20). The smaller the thermal resistance, that is, the smaller the temperature difference between the heat-receiving surface 42 and the condenser 11, the greater is the heat transfer rate from the evaporator 10 to the condenser 11. Consequently, the cooling ability is improved.

With the conventional loop heat pipe, the internal pressure in the evaporator increases as the quantity of heat increases, and the gap between the evaporator case 1001 and the wick 1007 spreads as illustrated in FIG. 1C. In this situation, the thermal resistance increases, and the cooling ability is impaired. In contrast, the loop heat pipe 1 of the first embodiment can maintain the cooling ability at the satisfactory level (by keeping the thermal resistance low). This is because the thermal contact between the dividing wall 51 of the evaporator case 40 and the wick 47 is maintained in the satisfactory state even if the temperature of the evaporator rises along with the increase in the quantity of heat generated from the electronic component.

FIG. 8A and FIG. 8B illustrate a first modification of the evaporator of the first embodiment. In this modification, an outer wall 63 (e.g., the top wall 63) of the evaporator that defines the pressure adjusting chamber 60B is made thinner than the dividing wall 61 separating the vaporization chamber 60A and the pressure adjusting chamber 60B. For example, the thickness of the dividing wall 61 is 2 mm, and the thickness of the top wall 63 of the evaporator case 40 is 1 mm. In the non-operating state, there is no deformation of the pressure adjusting chamber 60B occurring as illustrated in FIG. 8A. In operation (during heat absorption), the pressure adjusting chamber 60B expands as illustrated in FIG. 8B. Because the outer wall (top wall) 63 is made thinner than the internal dividing wall 61, the outer wall 63 swells outward (toward the atmosphere) due to the increased pressure of the vapor flowing into the pressure adjusting chamber 60B through the pressure adjusting hole 65, while little deformation occurs in the internal dividing wall 61. This arrangement is advantageous to maintain the adhesion between the internal dividing wall 61 and the wick 47 constant. Although in FIG. 8A and FIG. 8B, the thickness of the outer wall 63 is set half the thickness of the dividing wall 61, the invention is not limited to this example. The outer wall 63 is designed with an appropriate thickness as long as the outer wall 63 is deformable without affecting the shape of the dividing wall 61. The thickness of the outer wall 63 can be set to one fifth to two third of the thickness of the dividing wall 61, depending on the type of the working fluid used in the loop heat pipe 1.

FIG. 9A and FIG. 9B illustrate a second modification of the evaporator of the first embodiment. In the second modification, the thicknesses of the top wall 73 and the internal dividing wall 71 of the evaporator 70 are similar to each other, but the dividing wall 71 is slightly curved toward the vaporization chamber 70A in which the wick 47 is provided. In the non-operating state, there is no deformation in the pressure adjusting chamber 70B as illustrated in FIG. 9A. In operation (during heat absorption), the pressure adjusting chamber 70B expands as illustrated in FIG. 9B. The outer wall (top wall) 73 of the evaporator case 70 swells outward due to the vapor flowing into the pressure adjusting chamber 70B through the pressure adjusting hole 75. Simultaneously, the internal dividing wall 71 also deforms toward the wick 47 so as to increase the curvature. A compressive force acts on the dividing wall 71 so as to press it against the wick 47. Consequently, adhesion between the dividing wall 71 and the wick 47 is enhanced and the cooling ability of the loop heat pipe 1 is improved.

According to the arrangements of the first embodiment, the cooling ability of the loop heat pipe 1 is improved and settled with a simple structure, and stable operation of electronic equipment is realized.

Second Embodiment

FIG. 10A and FIG. 10B illustrate an evaporator 80 according to the second embodiment of the invention, where FIG. 10A is a cross-sectional view along a direction of flow of the working fluid and FIG. 10B is a cross-sectional view taken along the A-A′ line of FIG. 10B. In the second embodiment, the evaporator 80 has a vaporization chamber (first space) 90A with a liquid supply path 86 and a second fluid chamber (second space) 90B with an airtight structure. The second fluid chamber 90B is filled with a second fluid 100 that has a saturated vapor pressure higher than that of the working fluid supplied to the vaporization chamber 90A at the same temperature. At least a portion of the second fluid 100 is in a liquid phase 100 b. Referring to the graph in FIG. 4, when ethanol is used as the working fluid, the second fluid can be selected from the group of ethanol, pentane, butane, ammonia and so on. The selected fluid is introduced in the second fluid chamber 90B with a portion thereof in a liquid phase. If the working fluid is pentane, then the second fluid is selected from the group of pentane, butane, ammonia and so on, and introduced in the second fluid chamber 90B with a portion thereof in a liquid phase.

In the example illustrated in FIG. 10A and FIG. 10B, the bottom face of the evaporator case 90 is the heat receiving face 82. The evaporator 80 is mounted over a heat generating component 20 such that the heat receiving face 82 comes into contact with the heat generating component 20 (such as an electronic component 20) to receive heat from the electronic component 20 (see FIG. 11A and FIG. 11B). A wick (a porous material) 47 is provided in the vaporization chamber 90A so as to be mechanically and thermally in contact with the inner surface of the vaporization chamber 90A. The working fluid 89 supplied via the liquid line 83 to the vaporization chamber 90A penetrates in the wick 47 and is vaporized by heat transferred from the evaporation case 40 to the wick 47. The vaporized fluid flows through the grooves 45 formed in the wick 47 into the vapor line 84. A portion of the second fluid encapsulated in the second fluid chamber 90B is also vaporized by heat in the evaporation case 90 during operation of the electronic component. In this state, a vapor phase 100 a and a liquid phase 100 b coexist.

Explanation is made of the exemplified structure of the evaporator 80 illustrated in FIG. 10A and FIG. 10B. The evaporator case 90 is a flat plate case with an entire height of 18 mm, a width of 60 mm and a length of 70 mm. A double chamber structure is employed in which the second fluid chamber 90B is provided on the top of the vaporization chamber 90A. The second fluid chamber 90B is an airtight space with the dimensions 66 mm length×56 mm width×1 mm height. The second fluid chamber 90B and the vaporization chamber 90A are separated from each other by a dividing wall 91 with a thickness of 2 mm. The inner dimensions of the vaporization chamber 90A are 66 mm length×56 mm width×11 mm height. The thickness of the walls of the vaporization chamber 90A is 2 mm on the whole.

The material of the vaporization case 90 and the dividing wall 91 is oxygen-free copper in the second embodiment. Conventional flat evaporators are often made of a rigid material such as stainless so as to be tolerant of the high internal pressure. In contrast, the evaporator of the second embodiment does not necessarily use a rigid material, as will be described below. Rather, a material with a higher thermal conductivity than stainless is used such that the temperature distribution of the evaporator case 90 becomes uniform. For example, aluminum alloy can be used for reducing weight.

The wick 47 arranged inside the vaporization chamber 90A is made of sintered nickel. The porous diameter is about 10 μm, and the porosity is about 50%. The outer dimensions of the wick 47 are 50 mm length×56 mm width×11 mm height. Especially, the height of the wick 47 is set precisely such that the wick 47 is held in the vaporization chamber 90A in close contact with the inner wall thereof. Fifteen grooves (vapor passages) 45 with a width of 1 mm and a depth of 2 mm are formed at a pitch of 3 mm in the top face and the bottom face (which come into contact with the ceiling and the bottom of the vaporization chamber 90A, respectively). In the center of the wick 47 is formed a liquid supply path 86 with a height of 3 mm, a width of 40 mm and a length of 40 mm to take the working fluid 89 supplied from the liquid line 13 into the wick 47.

The vapor line 84 and the liquid line 83 connecting the evaporator 80 and the condenser 11 (see FIG. 2) are copper pipes with an outer diameter of 6 mm, an inner diameter of 5 mm, and a length of 300 mm. The condenser 11 is also a copper pipe, like the vapor line 84 and the liquid line 83, with an outer diameter of 6 mm, an inner diameter of 5 mm and a length of 400 mm. Radiation fins are thermally connected to the circumference of the pipe, and are cooled by the blast fan 12.

In the second embodiment, n-pentane is used as the working fluid 89. The boiling point of pentane under the atmospheric pressure is 36° C. The temperature of the working fluid 89 reaches around 50-70° C. during the operation of the loop heat pipe 1, and accordingly, the vapor pressure of pentane becomes at or above the atmospheric pressure. The second fluid chamber 90B contains 1 cc of butane serving as the second fluid in advance. Butane is introduced in the second fluid chamber 90B by evacuating the air from the second fluid chamber 90B and inletting only butane, using the same method as introducing the working fluid in the loop heat pipe 1. The vapor phase become dominant in the second fluid during operation of the heat generating component (i.e., the electronic component) 20, and at least a portion of the second fluid is in a liquid phase throughout the operating state and non-operating state.

FIG. 11A and FIG. 11B are schematic diagrams for explaining an advantage of the second embodiment. When butane is used as the second fluid 100, the saturated vapor pressure of butane is higher than that of n-pentane used as the working fluid 89 at the same temperature. There is no deformation in the second fluid chamber 90B in the non-operating state. Assuming that the temperatures on the working fluid side (in the vaporization chamber 90A) and the second fluid side (in the second fluid chamber 90B) of the evaporator case 90 are substantially the same during operation, then the dividing wall 91 is pressed toward the lower pressure side, that is, toward the vaporization chamber 90A in which the wick 47 is provided. As the temperature rises, the pressure difference between the working fluid 90 and the second fluid 100 becomes large. As the temperature of the evaporator case 90 rises due to heat transferred from the heat generating component 20, the dividing wall 91 is brought into closer contact with the wick 47. In this state, the top wall 93 of the second fluid chamber 90B swells outward because the pressure difference between the second fluid chamber 90B and the atmospheric pressure is greater than the pressure difference between the vaporization chamber 90A and the second fluid chamber 90B. At this time, the dividing wall 91 also tends to swell toward the vaporization chamber 90A, and the adhesion between the dividing wall 91 and the wick 47 is enhanced.

FIG. 12A and FIG. 12B illustrate a modification of the evaporator 80 of the second embodiment. In the above-described example, the thickness of the dividing wall 91 is 2 mm, which is the same as the thickness of the evaporator case 90. In the modification, the thickness of the dividing wall 91 a separating the vaporization chamber 90A and the second fluid chamber 90B of an evaporator 80 a is made less than the wall thickness of the evaporator case 90, and it is, for example, 1 mm. With this arrangement, there is no deformation in the second fluid chamber 90B in the non-operating state (FIG. 12A), and the dividing wall 91 a is more deformable during operation or heat absorption. Consequently, the dividing wall 91 a and the wick 47 come into tight contact with each other under higher compressive force (FIG. 12B).

FIG. 13A and FIG. 13B are schematic diagram illustrating a structure in which the evaporator 80 of the second embodiment is mounted over a heat generating component. The evaporator 80 of the loop heat pipe 1 is placed, via thermal grease 21, over the electronic component 20 on a printed circuit board 30 and secured to the printed circuit board 30 using attachment screws 31. The amount (rate) of heat absorption of the evaporator 80 is about 60 W in the second embodiment. At this time, the condenser 11 (not shown in FIG. 13A and FIG. 13B) is cooled at room temperature (25° C.) using a blast fan 12 (90 mm diameter, 12-volt driving voltage). Heat transferred from the electronic component 20 to the evaporator case 90 vaporizes the working fluid 89 penetrating in the wick 47. Simultaneously, the second fluid with a saturated vapor pressure higher than that of the working fluid 89 and encapsulated in the second fluid chamber 90B also vaporizes. The dividing wall 91 is pressed against the wick 47 in the vaporization chamber 90A.

FIG. 14 is a diagram illustrating the cooling ability of the loop heat pipe 1 of the second embodiment, with a comparison example a loop heat pipe in which a conventional wick structure illustrated in FIG. 1A through FIG. 1C is incorporated. The horizontal axis of the graph represents the amount (rate) of heat generated by a heater (i.e., the electronic component), and the vertical axis represents thermal resistance [° C./W] which is determined by dividing the difference between the average temperatures of the evaporator 10 and the condenser 11 by the quantity of heat generated by the electronic component 20. The smaller the thermal resistance, that is, the smaller the temperature difference between the heat-receiving surface 82 and the condenser 11, the greater is the heat transfer rate from the evaporator 80 to the condenser 11. Consequently, the cooling ability is improved.

With the conventional loop heat pipe, the temperature of the evaporator rises as the quantity of heat increases, and the gap between the evaporator case 1001 and the wick 1007 spreads as illustrated in FIG. 1C. In this situation, the thermal resistance increases, and the cooling ability is impaired. In contrast, the loop heat pipe 1 of the second embodiment can maintain the cooling ability in the satisfactory state (by maintaining the thermal resistance low). This is because the thermal contact between the dividing wall 91 (or 91 a) of the evaporator case 90 and the wick 47 is maintained in the satisfactory state even if the temperature of the evaporator rises along with the increase in the quantity of heat transferred from the electronic component.

To support the above-described advantage, the deformation of a copper (Cu) evaporator case 90 of 56 mm width is calculated when using pentane as the working fluid. With the conventional structure illustrated in FIGS. 1A-1C, the difference between the atmospheric pressure and the internal pressure of the evaporator case is 0.2 MPa at the LHP operating temperature (near 70° C.) as illustrated in FIG. 4. Under this condition, the evaporator case expands and deforms outward by 95 μm. This state impairs thermal contact between the evaporator case and the wick and the thermal resistance increases. In contrast, when butane is introduced in the second fluid chamber 90B illustrated in FIG. 11, the internal pressure in the vaporization chamber 90A is lower than the internal pressure in the second fluid chamber 90B by 0.5 MPa. If the wick 47 is not arranged in the vaporization chamber 90A, the dividing wall 91 of the evaporator case 90 will swells toward the vaporization chamber 90A by 140 μm. However, because the wick 47 is provided in the vaporization chamber 90A, the dividing wall 91 is pressed against the wick 47 and tight contact is produced between the dividing wall 91 and the wick 47.

According to the comparison between the graphs of FIG. 14 and FIG. 7, it is understood that the evaporator configuration of the second embodiment can further improve the cooling efficiency, compared to the first embodiment.

In the first and second embodiments, the second space is provided only on the top of the evaporator case, opposite to the heat-receiving face, to define a double chamber structure because the top face has a large area size of a thermally conductive surface. However, the second space may be provided so as to cover at least one of the side walls of the vaporization chamber (first space). If the second space is provided so as to cover the top face and a pair of side faces of the vaporization chamber (first space), the double chamber structure is applied to three sides of the evaporator, except for the heat receiving surface. In this case, thermal adhesion between the wick and the vaporization chamber is further enhanced.

All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of superiority or inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 

1. A loop heat pipe comprising: an evaporator to vaporize a working fluid due to heat supplied from an external heat source; a condenser to cause the vaporized working fluid to condense; and connecting lines to connect the evaporator and the condenser in a loop, wherein the evaporator includes a first space having set of walls including a contact wall that comes into contact with the external heat source, a second space provided adjacent to at least one of the walls other than the contact wall, and a through-hole formed in a dividing wall separating the first space and the second space to allow the first space and the second space to communicate with each other.
 2. The loop heat pipe according to claim 1, wherein the second space is provided adjacent to a wall opposite to the contact wall.
 3. The loop heat pipe according to claim 2, wherein a vapor pressure of the working fluid is higher than an atmospheric pressure, and wherein a thickness of an outer wall separating the second space and the atmosphere is less than a thickness of the dividing wall between the first space and the second space.
 4. The loop heat pipe according to claim 2, wherein a vapor pressure of the working fluid is higher than an atmospheric pressure, and wherein the dividing wall between the first space and the second space is swelled toward the first space.
 5. The loop heat pipe according to claim 3, wherein the working fluid is selected from a group of pentane, butane and ammonia.
 6. The loop heat pipe according to claim 4, wherein the working fluid is selected from a group of pentane, butane and ammonia.
 7. The loop heat pipe according to claim 1, wherein a porous material is provided along an inner wall of the first space, and a flow path is formed in the porous material through which the working fluid supplied from one of the connecting lines passes.
 8. The loop heat pipe according to claim 1, wherein the evaporator is made of a material with a thermal conductivity higher than that of stainless.
 9. A loop heat pipe comprising: an evaporator to vaporize a working fluid due to heat supplied from an external heat source; a condenser to cause the vaporized working fluid to condense; and connecting lines to connect the evaporator and the condenser in a loop, wherein the evaporator includes a first space having a first set of walls including a contact wall that comes into contact with the external heat source, and a second space provided adjacent to at least one of said walls other than the contact wall and defined by a second set of walls, the second space being filled with a second fluid that has a saturated vapor pressure higher than that of the working fluid at a same temperature.
 10. The loop heat pipe according to claim 9, wherein at least a part of the second fluid is in a liquid phase when the loop heat pipe is not activated.
 11. The loop heat pipe according to claim 9, wherein a thickness of a dividing wall separating the first space and the second space is less than a thickness of an outer wall separating the second space from an atmosphere.
 12. The loop heat pipe according to claim 9, wherein a porous material is provided along an inner wall of the first space, and a flow path is formed in the porous material through which the working fluid supplied from one of the connecting lines passes.
 13. The loop heat pipe according to claim 9, wherein the evaporator is made of a material with a thermal conductivity higher than that of stainless. 